Petroleum is an indispensable source of energy; however, most petroleum is heavy or sour petroleum, meaning that it contains a high amount of impurities (including sulfur and coke, a high carbon petroleum residue). Heavy petroleum must be upgraded before it is a commercially valuable product, such as fuel.
Use of supercritical water is effective to upgrade heavy petroleum feedstock. However, supercritical water reactors generally include two types, downflow and upflow reactors, based on the direction the supercritical water and petroleum-based composition flows. In downflow supercritical reactors, heavy hydrocarbons fractions flow very quickly due to higher density, resulting in a shortened residence time (known as channeling). This may hinder upgrading due to a shortened residence time for reactions to occur with the higher carbon-containing molecules that tend to reside in the heavier fractions. Upflow supercritical reactors may also experience difficulties due to heavy fractions accumulating in the bottom of the reactor, which may affect the upgrading process and may plug the reactor.
To combat these deficiencies, catalysts may be used. Among various types of catalysts, water soluble or organic soluble catalysts may be used to provide increased contact with the reactants and improve temperatures and residence times. However, catalysts conventionally exhibit very low stability in supercritical water conditions. The harsh conditions of the supercritical reactants often cause breakdown of the catalyst and result in the formation of insoluble aggregates, which may become seeds for coke formation. Coke can plug the reactor, slowing or stopping the upgrading process.